As is well known, demands for a volume holographic digital data storage system that can store a large amount of data, such as data for a motion picture film, have been increasing and various types of holographic digital data storage system have been recently developed for realizing high density optical storage capabilities.
The volume holographic digital data storage system allows a signal beam having information therein to interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored in a storage medium made of an optical refractive crystal. The optical refractive crystal is a material which may react differently on different amplitudes and phases of the interference pattern.
Various holograms can be recorded in the storage medium by changing an angle of incidence of the reference beam (angular multiplexing) and/or by moving the storage medium to change a recording area (shift multiplexing), so that a great number of holograms of binary data can be stored in the storage medium on a page-by-page basis.
An example of the conventional apparatus for storing and reconstructing holographic data is described in U.S. Pat. No. 6,490,061 B1.
FIG. 1 depicts a block diagram of a conventional apparatus for storing and reconstructing holographic data, which includes a light source 100, a beam splitter 102, a shutter 104, a spatial light modulator (SLM) 106, a first and a second reflection mirror 108 and 110, a first to a third lens 112, 114 and 118, a holographic medium 116 and a charge coupled device (CCD) 120.
The light source 100 generates a laser beam. The beam splitter 102 splits the laser beam into a reference beam and a signal beam, wherein the separated reference and signal beams travel along two different optical paths. The first and second reflection mirrors 108 and 110 reflect the reference beam. Next, the reference beam is transferred to the holographic medium 116 via the first lens 112.
The shutter 104 stays open in a recording mode to allow the signal beam to be transferred to the SLM 106. At the SLM 106, the signal beam is modulated into binary pixel data on a page-by-page basis based on input data applied to the SLM 106. The modulated signal beam travels to the holographic medium 116 via the second lens 114.
The interference pattern of the modulated signal beam interfering with the reference beam is stored in a specific spot within the holographic medium 116, wherein the interference pattern is correspondent to the input data applied to the SLM 106.
In this case, the holographic data are multiplexed before being stored in the holographic medium 116 by using, e.g., the shift multiplexing and/or the angular multiplexing in a predetermined way. Hereinafter, for simplicity, the apparatus employing only the shift multiplexing will be described herein.
During a reconstruction mode, the shutter 104 remains to be closed so that a signal beam cannot be introduced to the holographic medium 116. Accordingly, only a reference beam is irradiated onto the holographic medium 116 from the first lens 112.
When a reference beam is irradiated onto a spot where an interference pattern is recorded, a first portion of the reference beam is diffracted by the interference pattern, so that recorded binary data corresponding to an image to be displayed on the CCD 120 via a third lens 118 can be reproduced on the page-by-page basis. Meanwhile, a second portion of the reference beam may pass through the holographic medium 116.
Next, in the CCD 120, the demodulated signal from the spot of holographic medium 116 is converted to an electrical signal, i.e., an original data.
Finally, a decoder (not shown) may decode the electrical signal into two binary values, i.e., “0” and “1”, by comparing, e.g., a light intensity of the electrical signal with a threshold value.
In order to reproduce the holographic data, a servo system such as a linear stage can be used. The servo system may have position data representing locations where interference patterns have been recorded. The servo system controls the locations where reference beams are to be irradiated by using, e.g., a DC servo motor. In case of the shift multiplexing, the servo system can find a spot where an interference pattern has been stored by, e.g., moving the holographic medium 116 based on the position data, so that a reference beam is to be irradiated onto the spot. If the reference beam is irradiated onto the spot, the holographic data recorded in the spot are reproduced, so that the CCD 120 can capture the holographic data. A reconstruction process such as above may be repeated in accordance with the position data by the servo system. In this way, however, the position data should include information on precise data locations and the servo system should control the locations in accordance with the position data in high accuracy.
Alternatively, in a recording mode, a specific mark can be recorded on each page together with an interference pattern corresponding to input data applied to the SLM 106 in the holographic medium 116. The specific mark can be used to represent the presence of the interference pattern in a reading mode. By using the shift multiplexing, reference beams are irradiated onto the holographic medium 116 in a predetermined way. In this case, if a specific mark is reproduced and then detected at a detector (not shown) when a reference beam is irradiated onto a spot, an interference pattern must have been recorded in the spot. At this time, since the holographic data recorded in the spot are also reproduced, the detector allows the CCD 120 to capture the holographic data. In this way, the servo system and the position data such as above are not required. However, surplus space for recording the specific mark is needed in the holographic medium 116. Therefore, a recording capacity of the holographic medium 116 may be decreased.